Timing based servo arrangements are employed with respect to linear tape media, for example, in magnetic tape systems, to provide track following capability. Recorded servo patterns comprise dual magnetic transitions called “servo stripes” arranged as pairs recorded at more than one azimuthal orientation across the tape media. The technology is discussed in the incorporated U.S. Pat. No. 5,689,384. Reading a servo pattern by a servo read head yields a sequence of pulses called “dibits”, where each dibit corresponds to the transitions at both edges of a servo stripe written on the magnetic medium. The timing between dibits that correspond to any pair of servo stripes with different azimuthal orientation thus varies continuously as the servo read head is moved in the lateral direction across the servo band. The pattern is read by a servo read head whose width is small compared to the servo band pattern, and servo head position is derived from the relative timing of pulses generated by the servo head reading the servo pattern as the tape is moved in the longitudinal direction. Position sensing with this system is achieved by deriving a ratio of two servo pattern intervals, one pattern interval comprising the timing between dibits corresponding to a pair of servo stripes having different azimuthal orientation, and the other pattern interval comprising the timing between dibits corresponding to a pair of servo stripes having the same azimuthal orientation. Thus, the position sensing is ratio dependent and is insensitive to tape speed.
A typical way to determine which servo stripe is being read is to arrange the servo stripes into servo frames, each frame having two sub-frames, each with two bursts of servo stripes arranged in patterns with different azimuthal orientations. A pattern interval between dibits in the servo signal is provided for servo stripes within a sub-frame, and another pattern interval is between sub-frames. Typically, frames and sub-frames are distinguished by having a different number of servo stripes in the bursts of one sub-frame as compared to the other sub-frame. As one example, the frames and sub-frames are easily distinguished by observing the counts of servo stripes in each burst, such as 5, 5, 4, 4, within a frame, the 5, 5 count bursts comprising one sub-frame, and the 4, 4 count bursts comprising the other sub-frame.
A drawback of asymmetric numbers of servo stripes in each frame is that the extra servo stripes result in longer frames, reducing the rate of generation of lateral position estimates from the servo signals, as the lateral position estimates are generated on a frame by frame basis.